1. Field of the Invention
This invention relates to methods and apparatus for stabilizing closed-loop power controllers which controllers utilize switching techniques.
2. Discussion of the Prior Art
Because of their high efficiency, switching amplifiers are widely used to control power, for example, as audio amplifiers and motor controllers. Switching amplifiers are distinguished from linear amplifiers in that they amplify by using devices which switch at a frequency higher than the analog frequency of interest with such switching characteristics related to the analog signal being amplified. These switching amplifiers can be classified many ways, but for the present purpose, they are classified as open-loop and closed-loop power controllers. Closed-loop power controllers are distinguished from open-loop power controllers in that they use some form of feedback. This feedback is normally used to enhance the output linearity and signal-to-noise ratio.
Switching amplifiers normally use a passive low-pass filter between the power switching devices and the load to separate the lower frequency information contained in the switched output from the higher frequency switching energy. For example, when a switching amplifier is used to drive a speaker with audio signals, the passive filter would be designed to pass the highest audio frequency of interest while attenuating the switching energy and other high frequency noise to prevent electro-magnetic interference to other equipment. In this case a higher order low-pass filter would be more desirable than a simpler low-pass filter, because of the greater attenuation that would occur to the unwanted higher frequencies.
Another advantage of the passive filter is its attenuation of the error components outside of the frequency band of interest as seen at the means that controls the switching circuits. This process improves the analog output signal-to-noise ratio and linearity. Often, in addition to the passive filter, additional filtering, as described in pending parent U.S. patent application, Ser. No. 08/007,52l, is added elsewhere in the loop to enhance the linearity and signal-to-noise ratio of the analog output. In all cases the total amount of filtering or phase shift in the loop must be controlled to prevent instability.
The point from which feedback can be taken varies considerably. Closed-loop power controllers can take feedback from the output of the power switching devices as in U.S. Pat. Nos. 3,294,981, and 5,218,315, from the output of the passive filter (as in the above identified pending patent application), from the output of one of the stages of the passive filter as in U.S. Pat. No. 4,178,556), or from a combination of the above as described in U.S. Pat. No. 4,456,872.
In the cases where feedback is taken after one or more stages of passive filtering, it might be necessary to reduce the phase shift caused by the passive filter, either because the passive filter provides too much phase shift to the closed loop or because it is desirable to add an additional filter with more desirable characteristics. For example, the use of feedback originating after a fourth-order passive filter would normally cause instability and result in undesirable oscillations in the amplifier. Even if the desired output passive filter would not by itself cause instability in the closed loop, it might be desirable to add another complex low-pass filter, such as an active low-pass filter, in the closed loop at a point other than between the power switching networks and the analog output. Such a complex low-pass filter could have characteristics not easily obtained with a passive filter at the output. However, this complex low-pass filter, along with the output passive filter, might have so much phase lag as to cause instability of the closed loop. It is possible to bypass the output passive filter and provide feedback from the output of the power switching networks, but this method might undesirably reduce the switching amplifier's output impedance and damping factor.
Additionally, a switching amplifier's best linearity and signal-to-noise ratio occurs when the gain and phase characteristics of the closed-loop system cause the amplifier to operate near instability. Under these conditions of near instability, it is possible for the amplifier to become unstable during a power-on condition or because of an abnormally large signal or noise. Once the amplifier enters the unstable state of large signal oscillation, then one or more normally linear amplifiers are operating part of the time non-linearly and the closed loop system might not return to stability. Accordingly, there is a need to return an amplifier from an unstable state to a stable operating condition.
In view of the foregoing, it is an object of the present invention to provide high-efficiency, stable power controllers with enhanced signal-to-noise ratios and high linearity.